The purpose of this study is to evaluate the safety and effectiveness of vilazodone for the treatment of major depressive disorder versus citalopram. Doctors want to determine if vilazodone is effective for the treatment of major depressive disorder in those who have not responded to generic selective serotonin reuptake inhibitors (SSRI), which is a class of anti-depressant drugs such as Prozac, Lexapro, Paxil, or Zoloft. Both vilazodone and citalopram have been approved for the treatment of major depressive disorder. This research is being done because the researchers want to find out if vilazodone works in reducing the symptoms of depression significantly more than a generic SSRI.